There exist interactive display screens on most of the electronic devices for displaying various data, curves, graphs, images and the like on the display terminal (for example, CRT (cathode ray tube) or LCD (liquid crystal display)) through the cooperation of software and hardware. Conventional display techniques map a display terminal onto a two-dimensional array (which corresponds to a physical device called video memory), each pixel on the display terminal is mapped onto an element of the two-dimensional array, whose value is the color code of the pixel, and the ordinate and abscissa of the pixel are the two indices of the two-dimensional array. That is, the pixel with indices (x, y) on the display terminal is mapped onto the element A(x, y) of the two-dimensional array. The system calculates the values of the two-dimensional array based on the graph to be displayed and writes them into corresponding positions of the video memory and then the display driving module reads the display data out of the video memory and transforms them into pixel matrix information, and finally displays the picture on the display terminal. A window refers to a particular region opened on the display terminal for displaying, specific contents, such as a menu, dialogue boxes, charts, images and the like. A window corresponds to a set of display data stored in corresponding positions of the video memory. To display a window is to write display data of the window into the corresponding positions thereof in the video memory.
In the above mentioned display architecture, the display terminal is in one-to-one correspondence with the video memory. Every updating of the display screen is in correspondence with the updating operation of the display data in the video memory. In case of frequently opening and closing of certain windows, it implies the frequent write-in of display data into the video memory and thus a relatively high bandwidth is required. Practically, however, many of such write-in operations into the video memory are unnecessary. For example, a menu, dialogue box and the like are frequently required to the displayed on the display terminal. When some instructions are entered by the user, next menu or dialogue box may overlap cut ones. In reverse, when other instructions are entered by the user, the overlapping menus or dialogue boxes will disappear level by level, lastly the original display will recover on the display terminal. The physical operations corresponding to the above mentioned display variation processes on the display terminal are as follows: display data of the menus or dialogue boxes are written into the video memory in order. When a certain menu or dialogue box disappears, the display data overlapped by it are re-written into the video memory. Since the portion on the display terminal overlapped by the menu or dialogue box remains the same when it pops out, when the menu or dialogue box disappears, it is obviously an unnecessary operation to re-write the same data into the video memory. This causes a low efficiency of the display of windows and the consumption of extra processing resources and memory bandwidth. As for such types of devices as the desk top computers, such kind of consumption may be negligible because the processing capability of CPU and the bandwidth are sufficiently high. However, as for the majority of the embedded systems with limited resources, increasing the processing capability of CPU and the memory bandwidth means significant cost.